Issue #1139908 by bojanz, Damien Tournoud, fgm: batch requests

- also: added bojanz' MigrateDestinationResEntityAPI
- fixed wording to core standard: entity_type vs entity_name, bundle vs bundle_name
- API updated for batch mode
- no longer issue a count() RPC call on rewind()
- simplified flattening and make it more languge-resilient